1. Field of the Invention
The present invention relates to an electrical storage apparatus for use in an auxiliary power supply and relates, in particular, to an electrical storage apparatus for use in an auxiliary power supply for supplying an electric power from an electric storage device upon voltage drop of the main power supply.
2. Description of the Related Art
In recent years, a car (hereinafter referred to as a vehicle) equipped with an idling stop function to stop driving the engine at the timing of a stop for consideration for the environment and improving the fuel economy is put on the market. In such a vehicle, the battery voltage is temporarily lowered when a starter that intermittently consumes a large current is driven during use. This has also resulted in a lowered supply voltage to the loads of audio devices, a car navigation device and so on, possibly leading to unstable operation. Moreover, there have been proposed various vehicle braking systems developed from the conventional mechanical hydraulic control to an electrical hydraulic control with regard to the braking of the vehicle, and it has been possible that the load of the vehicle braking circuit or the like has become inoperable when the battery fails.
As a countermeasure against the above problems, an electrical storage apparatus as an auxiliary power supply for supplying a sufficient power to the loads also at the timing of temporary battery voltage fall and supplying a power to the vehicle braking system at the timing of battery failure is proposed in, for example, Japanese Registered Utility Model No. 2565018. FIG. 20 is a circuit diagram of the electrical storage apparatus of the prior art disclosed in the document. In FIG. 20, the part enclosed by the dashed lines corresponds to the circuit part disclosed in the Japanese Utility Model No. 2565018.
First of all, the circuit part enclosed by the dashed lines will be described referring to FIG. 20. A switch 103 as an ignition switch is connected to a battery 101 corresponding to the main power supply. When the switch 103 is turned on at the startup time of the vehicle, the power of an output voltage VCC is supplied to the entire vehicle via a diode 105 connected in series to the switch. On the other hand, the output voltage of the battery 101 is branched to the loads of a clock, a semiconductor memory and the like that need to be consistently driven regardless of the use of the vehicle, so that an output voltage VDD is consistently supplied to the loads via a diode 107 and a resistor 109. Moreover, a capacitor 111 as an auxiliary power supply is connected so that the output voltage VDD is maintained even when the battery 101 is removed for replacement or another purpose. With this arrangement, the capacitor 111 supplies a power, and therefore, the clock, the semiconductor memory and so on can be continuously driven.
Reference is next made to a case where the circuit constructed as above is applied as an auxiliary power supply for an idling-stop car, a vehicle braking system or the like with reference to FIG. 20 that shows the circuit diagram of the electrical storage apparatus of the prior art. That is, the output voltage VCC is connected to the load 113 as it is, and the output voltage VDD including the output voltage of the capacitor 111 is connected to the load 113 via a switch 115 and a diode 117 interlocked with the switch 103 as indicated by the oblique dashed lines. This therefore corresponds to the powers of two systems connected to the load 113.
The operation of the auxiliary power supply will be described next. It is noted that the switch 103 and the switch 115 are described on the basis of the configuration in which the two are interlocked with each other as shown in FIG. 20. Moreover, the capacitor 111, which is always connected to the battery 101, is therefore in a fully charged state.
When the switch 103 and the switch 115 are turned on in this state, the output voltage VCC is supplied to the load 113 when the output voltage VCC of the battery 101 is normal. Because the two of the diode 107 and the diode 117 are connected in series on the output voltage VDD side, a voltage drop becomes larger than on the output voltage VCC side. As a result, the output voltage VDD is not outputted, and the output voltage VCC is preferentially supplied to the load 113.
In this case, assuming that the output voltage VCC of the battery 101 becomes lower than the output voltage VDD due to a failure or the like of starter driving or the battery 101, then the voltages across the diode 105 and across the diode 117 are each reversed. Therefore, the diode 105 is turned off, and the diode 117 is turned on. As a result, the output voltage VDD of the capacitor 111 is supplied to the load 113. By the operation as described above, the output voltage VDD of the capacitor 111 is automatically supplied to the load 113 by the diode 105 and the diode 117 even if the voltage of the battery 101 is lowered, and therefore, the driving is not stopped.
Although the load 113 can actually be continuously driven by the electrical storage apparatus as described above even when the voltage of the battery 101 falls, the diode 105 and the diode 117 need to be reliably switched over when the voltage of the battery 101 falls particularly in a case where the apparatus is applied to a vehicle. However, since no judgment of the failure of the switchover circuit part can be made in the prior art configuration, there has been a problem that no sufficient reliability has been able to be obtained.